There is known, for example, an inkjet recording apparatus as an image forming apparatus such as a printer, a facsimile or a copy machine apparatus. An inkjet recording apparatus performs recording by discharging ink droplets from a recording head onto a recording medium such as a recording paper (hereinafter, simply referred to as a “paper” but material is not limited to a paper). The inkjet recording apparatus is capable of recording a fine image at a high speed with advantages such as a low running cost, a low noise and an easy color image recording using multi-color ink.
In such an inkjet recording apparatus, it is required to increase a positional accuracy of landing positions of ink droplets on a paper so as to improve an image quality. There is known, such as disclosed in Japanese Laid-Open Patent Applications No. 4-201469, No. 9-254460 and No. 2000-25249, an inkjet recording apparatus that uniformly charges a conveyance belt to attract a paper by an electrostatic force to maintain a distance between a recording head and the paper constant, and to prevent an offset in a position of the paper by accurately controlling a paper feed, and to prevent a lift of the paper so as to prevent jamming and contamination of the paper due to a contact between the paper and the recording head.
However, it is known that when a conveyance belt is uniformly charged at a positive voltage to attract a paper by an electrostatic attraction force, ink droplets injected from a recording head are influenced by an electric field such that a trajectory of ink droplets is deflected, which causes offsets in landing positions of the ink droplets on the paper and a reverse flow of ink mist toward the recording head. In order to prevent offsets in landing positions of ink droplets and reverse flow of ink mist, there is known a charging method such as disclosed in Japanese Laid-Open Patent Application No. 2000-25249, in which ink droplets being injected is prevented from being influenced by an electric field by weakening a potential of a surface of a paper by applying an electric charge having a polarity opposite to a charge of a conveyance belt, of which surface is uniformly charged, on an upstream side of a recording head in the conveyance direction. Additionally, the paper is cause to be attracted by the conveyance belt by an electrostatic attraction force by weakening a potential of the surface of the paper having the same polarity as the surface of the conveyance belt.
Further, as a charging method of a conveyance belt, there is known a method such as disclosed in Japanese laid-Open Patent Application No. 2003-103857, in which an alternating charge pattern is formed on the surface of the conveyance belt by applying positive and negative charges alternately onto the surface of the conveyance belt by causing a voltage applying means being brought in contact with the surface of the conveyance belt.
By the way, in order to accurately negate the surface potential of a recording medium as is disclosed in Japanese Laid-Open Patent Application No. 2000-25249, it is necessary to adjust an amount of electric charges applied to a recording medium in accordance with the surface potential preferably at multiple levels since the surface potential of a recording medium changes with a material the recording medium and an environment of use of an image forming apparatus.
Means such as a sensor can be provided for accurately measuring a surface potential or a surface resistance of a recording medium, but such means causes the apparatus to become more complex and causes a problem of an increase in the cost and power consumption.